Chalcopyrite (CuFeS.sub.2) is the most common ore of copper. A substantial part of world copper production is derived from this mineral, solely by pyrometallurgical treatment subsequent to flotation enrichment of the mineral. Recovery of the copper is effected in the following steps: roasting, matte smelting and converting. In each of these three steps sulfur dioxide is produced which cannot be discharged directly into the atmosphere and is therefore customarily used for the manufacture of sulfuric acid. But, as the manufacturing costs exceed those of other processes, the sulfuric acid so recovered can be sold only at an economic loss. Efforts have thus been made to recover the sulfur contained in the ore in the elemental form or to separate it as a water-insoluble compound. Several hydrometallurgical processes are available for this purpose.
From available literature on the subject, it is known that chalcopyrite is insoluble in the usual acids and is attacked only by ferric salt solutions. One method provides for the chalcopyrite concentrate to be ground to a particle size of less than 40 .mu.m, which is then leached in boiling concentrated ferric chloride solution. After leaching two hours, most of the copper has been extracted from the mineral. However, the further processing of the copper chloride solution, having a very high iron content, is difficult and uneconomical.
Bacterial leaching has also been successfully employed for extracting copper, particularly from strip mine deposits, in which the minerals are admixed with large amounts of worthless rock or gangue. But the very slow rate of reaction and the high capital costs of the plant required, make this method unsuitable for treating concentrates.
According to another known leaching process utilizing ammonia under pressure, approximately 95 percent copper is released from a chalcopyrite concentrate when leached for nine hours in an ammoniacal solution at an elevated temperature and an air pressure of 8 kg/sq.cm. However, all of the sulfur in the chalcopyrite is converted to ammonium sulfate, rather than the desired elemental sulfur, which precludes the economic adoption of the process on a large scale.
A process based on acid pressure leaching must be employed to permit the recovery of the major part of the sulfur in elemental and thus saleable form. One prior art proposal provides for the flotation concentrate to be ground to a particle size of less than 40 .mu.m and for the subsequent leaching operation to be carried out for three hours with a stoichiometric excess of concentrate to acid of 25 to 50 percent, within a temperature of 120.degree.C. and an oxygen pressure of 35 kg/sq.cm. The surplus undissolved concentrate, subsequently separated from the sulfur recovered by a necessarily complicated procedure, must be returned to the grinding and leaching processes. Any increase in the rate of reaction by raising the temperature is limited by the low melting point of sulfur. If this is exceeded, pellets of tough sulfur and concentrate are formed, making the process difficult to perform and reducing the yield of copper. All of the proposals known to me for the hydrometallurgical extraction of copper from chalcopyrite concentrates have the disadvantage of requiring the use of unduly high temperature and pressure and a long period for leaching.
In addition to raising temperatures, a known means of accelerating the reaction is to increase the area of reaction. As a rule, conventional ball milling over a period of several hours yields a desirable particle size of less than 40 .mu.m. By way of example, the following particle size distribution can be obtained when grinding a chalcopyrite concentrate: 45% with a particle size of less than 3 .mu.m, 49% with a particle size of 3 to 10 .mu.m and 6% with a particle size of 10 to 40 .mu.m. However, comminution on this scale does not significantly improve leaching performance as a particle size of considerably less than 3 .mu.m is desirable.
A type of ball mill, known as an attritor or attrition grinder, is capable of comminuting solids to much finer particle sizes. Machines of this kind are widely employed in the preparation of colored pigments and pharmaceuticals. Such machines have generally not found application in the processing of industrial metallic minerals, inasmuch as after such solids have been ground for a certain period, a state of equilibrium is attained between progressive comminution and particle agglomeration, as a result of which reduction ceases upon attainment of a specific particle size. As sulfide ores, particularly chalcopyrite, are highly hydrophobic, the resulting readiness with which agglomeration occurs does not permit appreciable reduction of particle size even by attrition grinding, and consequently no improvement in leaching performance is attained.